1. Field of the Invention
The invention relates in general to a method for estimating and removing inter-carrier interference (ICI), and more particularly to a lowly complicated method for estimating and removing ICI on a time domain.
2. Description of the Related Art
In recent years, orthogonal frequency division multiplexing (OFDM) communication technology has been widely used in communication system. In order to transmit data under wireless channel environment, the symbol time is prolonged, and a cyclic prefix (CP) is added to resolve the multi-path delay spread of wireless channel. However, when the symbol time is longer, the system becomes very sensitive to carrier frequency deviation. In a moving environment, the wireless channel environment is time-variant. When the speed is too high, the wireless channel becomes not fixed within a symbol time. Furthermore, the time-variant channel will damage symbol orthogonality, generate Doppler spread in frequency domain and incur inter-carrier interference (ICI) at the receiving end.
Many methods for removing ICI have been provided to resolve the above ICI problem. These methods are mainly divided into two categories. The category I methods remove ICI for a symbol in two stages. Stage I is mainly for estimating the time-variant channel, and stage II is for removing the interference. By means of specially designed time domain pilot signals or frequency domain pilot signals, the time-variant channel is estimated, the carrier value is detected, and the interference is removed according to a linear or a non-linear symbol detecting method.
The other category is called ‘inter-carrier interference self-cancellation scheme’. According to the inter-carrier interference self-cancellation scheme, the transmitting end modulates the to-be-transmitted original carrier to a plurality of adjacent and alternated carriers with opposite polarities. Each original carrier is mapped to a group of polar and anti-polar carriers. During reception, each polar carrier and anti-polar carrier are respectively multiplied by a positive sign and a negative sign and combined, and then the inter-carrier interference will be self-cancelled. However, as each original carrier corresponds to a group of anti-polar carriers, the utilization efficiency of the frequency spectrum will deteriorate severely.